Which of the following best describes the autoregulatory mechanism that maintains constant organ blood flow despite changes in perfusion pressure?

• A Increase in sympathetic tone to arterioles when pressure falls
• B Release of renin from juxtaglomerular cells in response to reduced pressure
• C Atrial natriuretic peptide release increasing natriuresis
• D Intrinsic myogenic response: arteriolar smooth muscle contracts when stretched by increased pressure ✓

Explanation

Autoregulation of blood flow in organs such as the brain, kidney, and heart is primarily mediated by the myogenic response: when transmural pressure rises, vascular smooth muscle is stretched and depolarizes, opening voltage-gated Ca2+ channels and causing contraction, thereby increasing resistance and normalizing flow. Metabolic autoregulation (local accumulation of CO2, adenosine, K+) also contributes, particularly in the heart and brain. The renin-angiotensin system operates on a longer time scale and is not the primary autoregulatory mechanism.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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